The present invention relates to the field of lubricants and protective coatings. More particularly, the invention relates to lubricants and protective coatings incorporating fluoropolymer particles.
Because of their remarkably low coefficient of friction as well as their high level of inertness, fluoropolymers, such as polytetrafluoroethylene, have gained widespread acceptance for reducing surface friction and as protective coatings. In particular, polytetrafluoroethylene (PTFE) has been used on various types of surfaces from the familiar frying pan to rubbing parts or valves in complex apparatus, such as artificial human hearts. Unfortunately however, the cost of either making entire parts from PTFE or applying PTFE coatings to existing surfaces is relatively expensive.
Considerable research has been carried out involving the incorporation of minute solid fluoropolymer particles in grease and oil based lubricants to thereby achieve some of the benefits of the fluoropolymers in a fluid medium. In particular, several engine oil additives which include dispersed particles of fluoropolymers are disclosed in the patent literature. For example, U.S. Pat. No. 3,933,656 to Reick, teaches a modified lubricant for an internal combustion engine which comprises a major amount of a conventional motor oil, with a minor amount of of submicron size PTFE particles, and a neutralizing agent to stabilize the dispersion to prevent agglomeration and coagulation of the particles. In this and several other patents, the theory is proposed that the lubricating action of PTFE containing medium is enhanced by virtue of the fact that the PTFE particles somehow become attached to the surfaces, particularly within the pores, of the engine thus lubricated, thereby creating a renewable coating of PTFE.
Naturally, substantial effort has been expended in this field to optimize the effectiveness of these PTFE containing lubricants as well as to minimize the problems associated therewith. In particular, much has been done to prevent the otherwise inherent problems of agglomeration, coagulation, and settling of the PTFE particles. For example, British patent application No. 2,090,284 teaches the method of coating the PTFE particles with a "buoyant" carrier with a relatively low specific gravity, after which the coated PTFE particles are dispersed in a heavier oil, i.e. one with a higher specific gravity such as mineral oil. In this way, the PTFE particles which have an even higher specific gravity are "floated" in the oil. For examples of other methods of obtaining a dispersion of PTFE particles in a carrier lubricant see U.S. Pat. Nos. 4,127,491; and 4,396,514.
One limitation with many of the PTFE containing liquid lubricants involves the fact that the oils and greases used to effect good dispersion of the particles have been of relatively high viscosity. These high viscosity oils, and greases although well suited for applications such as in crankcases or wheel bearings for automobiles, are undesirable for other applications such as for lubricating weapons or for coating exposed surfaces for corrosion protection. Also, the high viscosity lubricants are difficult if not impossible to apply in a spray format, particularly without the use of a high pressure aerosol container. Furthermore, the higher viscosity oils display a more limited amount of surface penetration.
U.S. Pat. No. 4,333,840 to Reick, discloses a "hybrid PTFE lubricant" wherein the viscosity of a PTFE containing medium was lowered by blending the original PTFE oil with a second oil of lower viscosity. This patent recites the advantage that the lower viscosity PTFE oil is better suited for use with weapons, particularly as a lubricant and corrosion protector.
Although the hybride PTFE oil disclosed by Reick has a lower viscosity and may therefore be better suited for certain applications such as weapons, it also faces certain limitations. In particular, because the oil is now of a lower total viscosity, its retention on surfaces will likely be limited. That is, the lighter oil carrier for the PTFE tends to be more easily removed from the surfaces to which it is applied.
In the copending application, Ser. No. 722,724, by the same inventor and of which this application is a continuation in part, at least a partial solution to some of the above problems is disclosed. In that application, the inventor disclosed an improved fluoropolymer surface penetrating lubricant wherein a fluoropolymer containing lubricant, such as a mineral oil with ground and sintered particles of polytetrafluoroethylene (PTFE) dispersed therein, is diluted with a solvent, such as the 1,1,1-trichloroethane based solvent produced by THE DOW CHEMICAL COMPANY under the tradename of "Chlorothene VG". The resultant composition is better suited to apply to surfaces as it has a temporarily lowered viscosity. The solvent is intended to evaporate after the composition is applied, thus leaving a thin, fairly viscous film of fluoropolymer containing lubricant on the surface. It was also noted that in this diluted format, the fluoropolymer lubricant was better able to penetrate the pores of the surface to be coated, thereby achieving better attachment of the particles and also facilitating enhanced corrosion protection of the surface.
It was observed by the inventor that coating a surface with a fluoropolymer containing lubricant in this way, had the effect of lowering the coefficient of friction for the surface, particularly in connection with the fluid drag on the surface. Unfortunately, the full potential of this benefit was not realized by the compositions therein disclosed. In particular, it was found that the carrier lubricant medium for the fluoropolymer particles, e.g. mineral oil, which was left on the surface was detrimental to the reduction of fluid drag on the surface. That is, the oils used left a residual "oiliness" or tackiness to the surface which was counterproductive to the objective of reducing fluid drag on the coated surface. This was found even when the surface was wiped off to remove excess oil. In addition, the residual oil was detrimental in that dirt and other minute debris may adhere to it. As a result, the inventor found that, although the composition produced according to his co-pending application Ser. No. 722,724 has performed remarkably well in numerous other situations, it did demonstrate certain problems in this particular situation where the fluoropolymer coating is inteded to reduce fluid drag on the surface.